The interests on plasmons, associated with nanostructured metals, have remarkably increased in the past decade. Surface plasmon resonance (SPR) sensor is one of the successful applications, which is widely used in biomedical research. On the other hand, localized surface plasmon resonance (LSPR) is also widely studied in a broad range of applications. The distinct property of LSPR is a tailored and sharp absorption and scattering peaks depending on the shape and sizes of the metal nanostructures. LSPR technology also possesses a more superior thermal stability than SPR technology. Therefore, LSPR absorption is very stable in the range of room temperature to 200° C.
While the mechanisms of LSPR and SPR seem comparable, the fundamentals of these two technologies are quite different. SPR uses a two-dimensional metal nanostructure to create strong plasmon polariton, which propagates on the metal surface in an evanescent mode. SPR technology requires a special optical geometry and control to achieve highly sensitive detection. In contrast, LSPR occurs when the metal structure (0-dimensional structure) is much smaller than the wavelength of the light. There is a specific energy (i.e. wavelength) that makes the electrons in the metal resonate by absorbing the photon energy. Therefore, it appears with a strong absorption at specific wavelength and that the absorbance is sensitive to the refractive indices of the host matrix and the metal nanostructure.
Some existing technologies use plasmon induced hot electron detection using photodetector structures. The absorbed photons in metal nanostructure create “hot” electron-hole pairs which have high enough energy to be extracted through Schottky barrier. For example, as illustrated in FIG. 1, a nanoparticle based semiconductor 8 is illustrated. Semiconductor 8 includes nanostructures 6 and electrodes 4.
Semiconductor 8 uses a Schottky junction based on internal hot-electron emission to provide a photocurrent. In particular, the photons that are absorbed in a metal contribute to generating “hot electrons” and, if the hot electron energy is high enough, the hot electrons can overcome the Schottky energy barrier at the boundary between metal and semiconductor. Therefore, the hot electrons move to the semiconductor, resulting in photocurrent.
However, semiconductor 8 relies on making the nanostructures serve as one of the electrodes in a diode structure such that the electrical connection to the device is maintained. Semiconductor 8 and other similar existing technologies exhibit very low responsivity (A/W) or external quantum efficiency due to limited absorption in the thin metal structure and inefficient hot electron diffusion from metal to semiconductor. Another issue with semiconductor 8 and these existing technologies is that the Schottky barrier height severely limits the minimum amount of detectable light energy.
Further, surface plasmon based sensors are successfully used in various applications, since this optical phenomenon provides extreme sensitivity and robustness. Conventional SPR sensors consist of a thin metal surface or nanoparticle structure, excitation light source and detectors with controlled optical geometry. However, since conventional SPR sensors use multiple components and long optical path for better resolution, it is difficult to have multiplexing capability in a lab-on-a-chip device which has excitation source, detector and sensing elements all together.